Questions about new Ram w/ diesel

I'm about to take the plunge into the diesel world. Got my sights set on a 2014 Ram 3500 dually cc w/ AISIN trans. While BSing with the service manager, he said to be careful when adding DEF to the truck as older, stale DEF will gum up the injectors and cause costly problems. Never heard this and now getting cold feet. I have always worked on my own vehicles but never a diesel. I'm not planning on tinkering with this truck but are clogged injectors common on these vehicles? How much DEF will it actually use. SM said that 5.5 gallons will last 5k miles. True?
Finally what type of mpg would I expect pulling a 13k FW with 3.42 gears? What type of mpg for unloaded traveling?

Thanks

Buy the truck!!!! It will put a smile on your face that extends from ear to ear. DEF goes into a different tank that is totally dedicated to DEF. It does not mix with diesel prior to the engine nor does it pass through the engine's diesel injectors. It is injected downstream from the engine into the exhaust. Just keep fresh DEF if that is a concern. There are DEF shelf life tables based on temperature that you might be interested in.

A far bigger issue would be accidentially putting DEF into the diesel tank. That is a no no. Another thing to watch for is that not all green handles at fueling stations are diesel. Always check to see if it is diesel before putting in into the truck.